A synthetic diamond and a process for the production of the same

Abstract: A high purity synthetic diamond with very few impurities and crystal defects and low internal strains is provided, in which the nitrogen content is at most 10 ppm, preferably at the most 0.1 ppm, and the boron content is at the most 1 ppm, preferably at the most 0.1 ppm, or in which nitrogen atoms and boron atoms are contained in the crystal and the difference between the number of the nitrogen atoms and that of the boron atoms is at the most 1 x 10¹⁷ atoms/cm³. The strain-free synthetic diamond is produced by a process based on the temperature gradient method, which comprises using a carbon source having a boron content of at the most 10 ppm and a solvent metal having a boron content of at the most 1 ppm and adding a nitrogen getter to the solvent metal. The synthetic diamond can also be produced by a process based on the temperature gradient method, which comprises controlling the amount of a nitrogen getter added to a solvent metal and/or controlling the amount of boron added to a carbon source and/or a solvent metal so that the number of nigrogen atoms incorporated into the crystal is substantially the same as the number of boron atoms incorporated into the crystal during synthesis. Methods of measuring the strain in a synthetic diamond are based on the double or quadruple crystal method or on Raman spectroscopic analysis.

Description: This invention relates to a colourless, transparent, high purity and good crystallinity synthetic diamond, substantially free from impurities, crystal defects, strains, etc., and a process for the production of the same and also a method for measuring the strain in diamond. Diamond crystals can be applied to various uses such as heat sinks, dies for wire drawing, cutting tools for precision working, optical parts, laser windows, and anvils for producing ultra-high pressures, because of their high hardness and strength, excellent thermal conductivity and corrosion resistance and good transmittance of light. Naturally occurring diamonds, most of which are called Ia type, contain about 1000 ppm of nitrogen. The nitrogen in this natural diamond is distributed in the crystal in an aggregated form, resulting in a significant number of crystal defects and high internal strains. Additionally, there is absorption of light in the infrared range due to the presence of the nitrogen. Depending upon the variety of a rough stone, there is a large dispersion. Consequently, the applied use has been limited to heat sinks or tools. High purity natural diamond containing nitrogen impurities in an amount of at the most several ppm is called IIa type and such diamond constitutes an output of only about 2 % based on all rough stones. Since the natural diamond of IIa type contains a very small amount of impurities, is colorless and transparent and has superior transmittance properties, it has widely been applied to jewelery, optical parts and laser window materials. However, defects and strains still remain to a considerable extent in the interior part of the natural diamond because of the complicated growth processes which have occured in the interior part of the earth. With regard to the lattice strain, natural diamond suffers to a greater extent than synthetic diamonds containing nitrogen. Furthermore, natural diamond of IIa type suffers to such an extent from strains in the lattice, that the output quantity is small, resulting in high production costs. An ordinary diamond artificially synthesized under ultra-high pressure and high temperature is called type Ib and contains several hundreds ppm of nitrogen. Since the nitrogen is contained in the diamond crystal as an isolated substitutional impurity, the crystal is rendered yellow and is therefore less valuable as a jewel. In addition, the concentration of nitrogen in the diamond varies depending on the growth sectors and the nitrogen distribution is therefore largely uneven in the interior part of the crystal, thus resulting in greater strains in the crystal. It is known that when a nitrogen getter such as Al is added to a solvent metal during the synthesis of diamond, the nitrogen in the diamond can be removed to a level of about several ppm to obtain diamond of IIa type. When the nitrogen getter is added to the solvent metal, however, inclusions in large amounts ordinarily tend to be incorporated into the crystal, thereby decreasing the production yield of good quality crystal. Accordingly, the production cost of the synthetic diamond of IIa type is higher than that of the natural diamond of IIa type. Removal of nitrogen in the synthetic diamond is limited to about 1 ppm and the estimation of the diamond as a decorative article is approximately H to J by GIA scale (Japanese Patent Laid-Open Publication No. 88289/1977). Moreover, there is absorption in the ultraviolet range, due to the nitrogen incorporated into the crystal. As outlined above, a synthetic diamond crystal containing substantially no nitrogen and substantially free from inclusions or internal defects has hitherto been unknown. It is well known that elements such as Ti or Zr may be used as a nitrogen getter. When using such an element as a nitrogen getter, nitrogen can effectively be removed, but carbides such as TiC, ZrC, etc. are formed in large amounts in the associated solvent material and incorporated into the diamond crystal, so that good quality diamond can rarely be obtained. EP-A-0594994 relates to a semiconductor diamond and a process for the manufacture thereof. We have now succeeded in producing an inclusion free synthetic diamond of IIa type having a nitrogen content of at the most 10 ppm by using at least one element selected from Group IVa and Va elements having a high nitrogen removal efficiency as a nitrogen getter and simultaneously adding to a solvent metal a material capable of suppressing the formation of a carbide of a Group IVa element, a material capable of diffusing the carbide or a material capable of improving the activity of carbon in the solvent metal, so that inclusions are not incorporated into the crystal. However, several ppm of boron are still included in the crystal, so that there is an absorption of light, due to the boron, in the infrared range and also some strains and defects in the crystal. As described above, natural diamond has a number of defects and large strains in the interior part of the crystal. Natural diamond of IIa type contains less impurities, but is poor with regard to crystallinity, defects, strains, etc. Thus, natural diamond of IIa type tends to crack during working, when used in applications requiring strength, for example, an anvil for producing an ultra-high pressure, a compression cell for FT-IR, a laser window material, etc. Further, it cannot be used in applications requiring high crystallinity, for example, monochromaters, semiconductor substrates, etc. On the other hand, a synthetic diamond of IIa type has a greater crystallinity than natural diamond, but is deficient in other respects. For example, the working yield is low, the mechanical strength is lower than that of diamond itself and the synthetic diamond of IIa type also cannot be used in applications requiring high crystallinity, for example, monochromaters, semiconductor substrates, etc. The synthetic diamond crystal of IIa type of the prior art contains several ppm of boron and thus absorbs light in the infrared range, thereby rendering it unsuitable for optical applications. In addition, there are some defects and strains in the crystal. In the synthesis of diamond by the temperature gradient method, diamond is used as a carbon source. However, a commercially available diamond powder contains 10 to 1000 ppm of boron and natural diamond powder contains several tens to several hundreds ppm of boron with a large dispersion. In the synthesis of diamond using such a carbon source, several ppm of boron are present resulting in a blue color. Accordingly, there occurs an absorption due to boron in the infrared range and ultraviolet to visible range, which is obviously undesirable for optical applications. The concentration of boron in the diamond varies depending on the growth sectors and the boron distribution is therefore largely uneven in the interior part of the crystal. This is considered to be one reason in explaining why the crystallinity is not particularly good. Measurement of the strain in diamond has hitherto been carried out by a visual measurement using a polarizing microscope, but this method is quantitatively imprecise. There has been proposed a method for measuring the strain in diamond using Si (004) or Ge (004) commonly used as a first crystal in the double crystal X-ray diffraction method, but since the crystal interplanar spacing thereof is not the same as diamond, the FWHM of a rocking curve is significantly broadened (e.g. about 60 arcseconds) and, therefore, this method does not lead to a precise and quantitative estimation of strain. The present invention provides a high purity synthetic diamond with less impurities, crystal defects, strains, etc., and a proccss for the production of the same and also a method for measuring the strain in the synthetic diamond. As outlined above, when boron is substantially removed, a colorless, transparent and defect and strain-free crystal is obtained. However, in this case, very high purity raw materials (carbon source and solvent) are required and, consequently, problems on the supply and cost of the raw materials arise. It is an object of the present invention to provide a high purity synthetic diamond with few impurities and crystal defects and low strains, etc., whereby the disadvantages of the prior art can be overcome. It is another object of the present invention to provide a process for the production of a colourless, transparent, high purity and good crystallinity synthetic diamond substantially free from impurities, crystal defects, strains, etc. Accordingly, the present invention provides a synthetic diamond, in which the nitrogen content is at the most 10 ppm and the boron content is at the most 1 ppm and in which in an X-ray diffraction analysis by the double crystal method, in which a diamond crystal is used as a first crystal, the FWHM of the X-ray diffraction rocking curve is as the most 10 arcseconds for measurement using CuKα rays with an arrangement in parallel to the (004) plane. The present invention also provides a process for the production of a synthetic diamond as herein described by the temperature gradient method employing a carbon source, a solvent metal and a seed crystal, wherein the carbon source is separated from the seed crystal by the solvent metal and a temperature gradient exists between the carbon source and seed crystal at an ultra-high pressure and a high temperature, which comprises using a carbon source having a boron content of at the most 10 ppm and a solvent metal having a boron content of at the most 1 ppm and adding a nitrogen getter to the solvent metal. In an alternative aspect, the present invention further provides a process for the production of a synthetic diamond as herein described by the temperature gradient method employing a carbon source, a solvent metal and a seed crystal, wherein the carbon source is separated from the seed crystal by the solvent metal and a temperature gradient exists between the carbon source and seed crystal at an ultra-high pressure and a high temperature, which comprises controlling the amount of a nitrogen getter added to a solvent metal and/or controlling the amount of boron added to a carbon source and/or a solvent metal so that the number of nitrogen atoms incorporated into the crystal is substantially the same as the number of boron atoms incorporated into the crystal during synthesis. The accompanying drawings illustrate the principles and merits of the present invention. Fig. 1 is a schematic view of the structure of a sample chamber for the synthesis of a diamond crystal in one embodiment of the present invention, showing a carbon source (1), a solvent metal (2), a seed crystal (3), an insulator (4), a graphic heater (5) and a pressure medium (6). Fig. 2 is a graph showing the relationship between the amount of Ti or Al getter added and the nitrogen content in the crystal. Fig 3 is a schematic view of the cross-section of a crystal. Fig. 4 is a graph showing the relationship between the boron content and nitrogen-presumed content in the sector (111) and sector (100) and the absorption spectrum of ultraviolet to visible in each sector. Fig. 5 illustrates ultraviolet absorption spectra of diamond crystals synthesized by changing the amounts of Ti and Cu. Fig. 6 illustrates an estimation standard for the estimation (++, +, -) of metal inclusions in Table 1. The present invention has been developed to solve the above described problems and thus provides a strain-free synthetic diamond, characterized in that the nitrogen content is at the most 10 ppm, preferably at the most 0.1 ppm, and the boron content is at the most 1 ppm, preferably at the most 0.1 ppm, and in that in the X-ray diffraction by the double crystal method, a diamond crystal is used as a first crystal and the FWHM of the X-ray diffraction rocking curve is at the most 10 arcseconds, preferably at the most 6 arcseconds for measurement using CuKα rays with an arrangement in parallel to (004) plane. Advantageously, the FWHM of a peak at 1332 to 1333 cm -1 in the Raman spectrum of diamond, measured by means of an apparatus for Raman spectroscopic analysis with a resolving power of at the most 1 cm -1 is at the most 2.3 cm -1 , preferably at the most 2 cm -1 . Preferably, nitrogen atoms and boron atoms are contained in the crystal and the difference between the number of the nitrogen atoms and that of the boron atoms is at most 1 x 10 17 atoms/cm 3 . Herein, the unit of FWHM, arcseconds, is used as a measure for indicating the completeness of crystallinity. The theoretical value of FWHM of a complete crystal is 4.1 arcseconds. In the present invention, in particular, it is preferable to use, as the above described carbon source, graphite which has been subjected to a halogenizing treatment to reduce the boron content to at the most 1 ppm. In the present invention, in particular, the above described nitrogen getter is preferably at least one elememt from Group IVa or Va of the Periodic Table. In the present invention, in particular, the above described solvent metal is preferably one to which an element is added capable of preventing carbide formation of at least one element from Group IVa or Va of the Periodic Table. In the present invention, in particular, it is preferable to arrange a buffer material for stabilizing the initial growth of the crystal between the surface of a seed and solvent metal. In a particularly preferable embodiment of the present invention, diamond is synthesized at an ultra-high pressure and high temperature by the temperature gradient method, wherein the temperature and pressure at a sample section are reduced to normal temperature and pressure, and the reduction in the pressure is completed at a temperature in the range of from 300 to 1000 °C in the sample section. In order to solve the above described problems, the inventors have estimated the crystallinity of various diamonds by various means. During studies therefor, an X-ray diffraction method has been developed using, as a first crystal, a very high purity, strain-free synthetic diamond developed by the inventors. According to this method, a very precise and quantitative measurement of strain can be carried out. Thus, the inventors have attained such a new knowledge that the strain in a diamond crystal can precisely and quantitatively be estimated by the FWHM of an X-ray diffraction rocking curve measured by the use of a diamond crystal as the first crystal, by the FWHM of an X-ray diffracton rocking curve measured by the quadruple crystal method or by the FWHM of a peak at peak at 1332 to 1333 cm -1 in the Raman spectrum. As a result of these procedures, it is found that there are more strains in natural diamonds of Ia type and IIa type and much less strains in synthetic diamonds as compared with the natural diamonds. However, it is found that even in the case of the synthetic diamonds, there are more strains in those containing some or more of nitrogen or boron, and when the synthetic diamonds contain at most 0.1 ppm of impurities, strains are hardly found in the diamond crystal. That is, it is further found that strains are hardly found in the crystal when in the double crystal X-ray diffraction method, diamond crystal is used as a first crystal and the FWHM of the X-ray diffraction rocking curve is at most 6 arcseconds in the case of measuring by CuKα ray with an arrangement in parallel to (004) plane, or when the FWHM of a peak at 1332 to 1333 cm -1 in the above described Raman spectrum is at most 2 cm -1 . In the production process of the present invention, for the purpose of reducing internal defects or strains in a crystal, boron impurities in raw materials or solvent metal are decreased to such an extent as little as possible and 0.1 to 5 % by weight of a nitrogen getter is added to the weight of the solvent metal to adjust the nitrogen content and boron content in the crystal respectively to preferably at most 0.1 ppm. As the solvent metal, for example, there are used metals such as Fe, Co, Ni, Mn, Cr, etc. and alloys thereof. As the nitrogen getter, there is used a material having a reactivity with nitrogen without hindering the growth of a diamond crystal, for example, at least one metal selected from the group consisting of Group IVa and Va metals of Periodic Table, such as Ti, Zr, Hf, V, Nb, Ta, etc., and alloys of these metals. As the carbon source of the present invention, there is preferably used a high purity diamond powder having a boron content of at most 10 ppm, more preferably a high purity graphite subjected to a halogenizing treatment to substantially remove boron. The use of the latter is more effective. At the same time, it is effective to add an element capable of preventing formation of a carbide of an element selected from Group IVa and Va elements of Periodic Table, for example, Cu, Ag, Au, Zn, Cd, etc. in a proportion of 0.1 to 20 % by weight to the solvent metal. Moreover, it is very effective to arrange a buffer material consisting of an element selected from the group consisting of Al, Ni, Cu, Zn, Ga, Ag, Cd, In, Sn, Au, Ti and Pb, for example, a thin sheet of Al or Cu with a thickness of 0.01 to 0.5 mm, between the surface of a seed and solvent metal for the purpose of stabilizing the initial growing state of a diamond crystal. The strain due to incorporation of inclusions and the crystalline defects or strains due to unstable growth at the initial crystal growth can be reduced by employing these methods or means. In the technical field for the synthesis of diamond by the temperature gradient method, it is more preferable to release the pressure at an internal temperature of 300 to 1000 °C , preferably 400 to 800 °C , more preferably 500 to 600 °C after synthesizing diamond under the commonly used ultra-high pressure and high temperature, so that strain due to stress is not retained in the crystal. In the present invention, the synthetic diamond preferably contains nitrogen atoms and boron atoms in the crystal and the difference between the number of the nitrogen atoms and that of the boron atoms is preferably at most 1 x 10 17 atoms/cm 3 , more preferably at most 1 x 10 16 atoms/cm 3 . The inventors have made various efforts to solve the above described problems and consequently, have found that blueing of diamond crystal and lowering of the crystallinity thereof due to the presence of boron impurities from raw materials can be compensated by retaining nitrogen to some extent in the diamond crystal. Thus, the inventors have attained a colorless and trasnparent diamond which contains boron and nitrogen, but which is free from optical absorption of boron or nitrogen by controlling the amount of nitrogen removed, and a process for the production of the same. According to the present invention, it is found that the strain in the crystal due to boron is moderated and the crystallinity is improved. When using a nitrogen getter such as Al, moreover, it is difficult to substantially completely remove nitrogen in diamond. It is found in this case, however, that when boron is added to the crystal in a substantially similar amount to the amount of nitrogen which has not been removed and has been retained, a colorless and transparent diamond which contains boron and nitrogen, but which is free from optical absorption of boron or nitrogen is obtained. It is further found that the strain in the crystal due to nitrogen is moderated and the crystallinity is improved. This will now be illustrated in greater detail. When the synthesis is carried out in an Fe-Al type solvent, the reaction of Al (Al + N = AlN) is not strong, but the removal efficiency of nitrogen is low. In contrast, when adding Ti to a solvent, high quality and high purity diamond substantially free from inclusions can be synthesized when the nitrogen content is at most about 0.1 ppm. In a diamond synthesized by using a Ti getter, however, absorption of boron (content about 0.2 ppm) appears in the infrared or near infrared range, which does not appear in the case of an Al getter. It is assumed from this fact that absorption due to nitrogen and boron will disappear because boron acting as a p-type semiconductor (acceptor) and nitrogen acting as a n-type semiconductor (donor) form AD pair during synthesis of a crystal and are electrically neutralized. That is, it is considered that in the case of the synthesis with the Al getter having a low nitrogen removal effect, nitrogen is well electrically neutralized with boron taken in the crystal to reduce the absorption, whilst in the case of using a nitrogen getter, nitrogen is substantially completely removed and only an absorption due to the boron taken in the crystal appears. In order to confirm this effect, a syhthesis was tried by positively adding boron under such a condition that nitrogen remains in a relatively large amount. Consequently, an absorption (at 4.6 eV) of nitrogen of Ib type in the ultraviolet range is largely reduced in the sector (111) enriched in boron. (Experiments and Results) 1 ○ Diamond Synthesis by Addition of Boron To an Fe-type metal solvent are added Al in an amount of 0.83 atomic % and boron in variable amounts, i.e. of 15, 30 and 50 ppm. Synthesis of Diamond single crystal is carried out by an epitaxial growth on a seed crystal according to the temperature gradient method under synthetic conditions of a pressure of 5.5 GPa and a temperature of 135 °C. A high purity raw material containing no boron is used in the synthesis. Above all, a crystal synthesized by adding 30 ppm of boron is most colorless and transparent and accordingly, polishing is carried out in such a manner that the plane (110) be an observing plane. 2 ○ Observation of Crystal Cross-section Fig. 3 shows a schematic view of a crystal cross-section and Fig. 4 shows the boron content and estimated nitrogen content in each sector. The boron content is measured by comparison of a sample by SIMS with a previously prepared standard sample whose boron content is clarified by the ion implantation method. The nitrogen content is estimated from the graph of Fig. 2 showing the relationship between the amount of Ti or Al getter added and the nitrogen content in the crystal. In Fig. 2, the abscissa shows the amount of a getter added (atomic percent) and the ordinate shows the nitrogen content (ppm) in the crystal. ○ = Ti, ▵ = Al. Furthermore, the ultraviolet-visible absorption spectra in Sector (111) and Sector (100) are measured to obtain results as shown in Fig. 4. In Sector (111), in which boron and nitrogen are contained in substantially equimolar amounts, absorption of nitrogen of Ib type (at 4.6 eV) markedly small to give an absorption of IIa type. In Sector (100), in which boron is hardly contained, absorption of nitrogen of Ib type appears. The both show IIa type in the absorption in the infrared range. 3 ○ Consideration It is assumed that AD pair is formed since in the sector, in which boron and nitrogen are contained in substantially equimolar amounts, absorption of nitrogen of Ib type dissolved in the isolated dispersion type is decreased, as described above. In the present invention, it is desired that the number of nitrogen and boron atoms contained in diamond crystal are substantially the same. If the difference between the number of both the atoms is at most 1 x 10 17 atoms/cm 3 , a diamond crystal whose strains are more reduced as compared with natural diamond and synthetic diamond of Ib type of the prior art will be obtained. That is, there can be obtained a diamond crystal having more excellent crystallinity than that of the prior art, in which the FWHM of the X-ray diffraction rocking curve is at most 10 arcseconds, measured by an arrangement of a first crystal in parallel to (004) plane of diamond, or the FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum is at most 2.3 cm -1 . For example, seven to ten crystals are respectively prepared as to natural diamond of IIa type, nattural diamond of Ia type and synthetic diamond of Ib type of the prior art and subjected to the above described measurements to obtain respectively rocking curve FWHM of 300 to 3000 arcseconds, 200 to 600 arcseconds and 15 to 30 arc-seconds, and FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum of 2.15 to 3.5 cm -1 , 2.8 to 3.3 cm -1 and 2.4 to 2.6 cm -1 . Furthermore, it is more preferable to use a diamond crystal such that the difference between the number of nitrogen atoms and boron atoms is at most 10 16 atoms/cm 3 so as to give improved crystallinity. In this case, the diamond has such a good crystallinity that the FWHM of the X-ray diffraction rocking curve is at most 7 arcseconds, measured by an arrangement of a first crystal in parallel to (004) plane of diamond, or the FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum is at most 2 cm -1 and can thus be applied to uses needing high grade crystallinity, for example, monochromators, semiconductor substrates, etc. In this case, this diamond can also be applied to various optical parts or window materials because of being from absorption of light by impurities such as nitrogen, boron, etc. exclusive of the absorption by the diamond itself in the near ultraviolet to far infrared range and applied to uses for jewels because of being a colorless and highly transparent diamond crystal. In this case, the color grade of the diamond crystal is at least G color by GIA scale, corresponding to the highest level as an estimation of the diamond for jewels. It will clearly be understood how the diamond of the present invention is excellent from the fact that the well-known synthetic diamond of IIa type is of middle class represented by H to J in the same estimation (Japanese Patent Laid-Open Publication No. 88289/1977). Synthesis of the diamond as described above is carried out by controlling the amount of a nitrogen getter to be added to a solvent so that the amounts of nitrogen and boron taken in the crystal are approximately same by the number of the atoms, or controlling the amount of boron to be added to a carbon source or solvent. Specifically, the following procedures are employed. (1) The amount of a nitrogen getter added is controlled in such a manner that nitrogen is retained in the crystal in substantially the same amount as the amount of boron which is contained, as an impurity, in a carbon source and solvent raw material and taken in the diamond crystal. (2) A nitrogen getter is added to such an extent that the crystal growth of diamond is not hindered and boron or a boron compound is previously added to a carbon source or solvent so that boron is taken in the diamond crystal in substantially the same amount as that of nitrogen retained during the same time. A suitable procedure can be selected from these procedures considering the varieties of the carbon source or solvent (boron content), the nitrogen removal efficiency of the nitrogen getter, the degree of hindering the crystal growth of the nitrogen getter, etc. As the nitrogen getter added to the solvent in the present invention, there can be used Al or Group IVa or Group Va elements such as Ti, Zr, Hf, V, Nb, Ta, etc. In the case of using Group IVa or Group Va elements as the nitrogen getter, it is preferable to add a material capable of suppressing formation of carbides of these elements. For example, at least one element selected from Al, Ni, Cu, Zn, Ga, Ag, Cd, In, Sn, Au, Tl and Pb is added, as such a material, in a proportion of 0.1 to 20 weight % to the solvent metal. The following experiment is carried out based on this knowledge. (a) Experiment Ti and Cu were added to a metallic solvent and maintained at a pressure of 5.5 GPa and a temperature of 1300 to 1400 °C for several tens of hours in the temperature gradient method, thus growing a diamond crystal of 1 to 2 carats. The metallic inclusion of the solvent or other inclusions contained in the resulting crystal were observed by a microscope. When this crystal was polished in a thickness of 1 mm and subjected to measurement of the ultraviolet absorp tion spectrum, the amount of nitrogen was estimated by absorption at 4.6 eV. (b) Results and Consideration When Ti was only added to a metallic solvent, large amounts of inclusions were contained in the crystal, i.e. large amounts of impurities of ten and several µ m were contained in addition to the inclusion of the metallic solvent and a good quality diamond crystal was hardly obtained. When the cross-section of the metallic solvent was observed after the synthesis, foreign matters of several µ m to ten and several µ m were found, which was found to be TiC as a result of analysis by EPMA. It is assumed that fine impurities in the diamond crystal consist predominantly of TiC taken in the diamond crystal. The inclusion of the metal solvent in a large amount is probably due to formation of TiC in a large amount in the metal solvent, whereby supply of carbon is hindered. When Ti and further Cu are added, TiC observed in the solvent is size-reduced to 1 µ m or less and the amount thereof is largely decreased, resulting in decrease of the inclusions in the diamond crystal. It is assumed that TiC is decompsed or the formation of TiC is suppressed by the addition of Cu. Table 1 shows results of typical synthetic experiments. When Ti is added in an amount of at least 1.5 %, nitrogen is substantially completely removed and even when Cu is added in an amount of 1 to 3 %, this state is not changed. Fig. 5 shows ultraviolet absorption spectra of diamond crystals synthesized with changing the amount of Ti an Cu added and when adding 1.5 % of Ti, the absorption by nitrogen is hardly found. The addition of Cu in substantially the same amount as the amount of Ti results in large reduction of the inclusion of the solvent and incorporation of fine impurities, whereby a good quality diamond crystal can be obtained in stable manner. The standard (++, +, -) of estimation of the metal inclusion in Table 1 is shown in Fig. 6. Added Amount (wt %) In Diamond Crystal Inclusions Ti Cu N Content (ppm) Metal Fine Impurities 0.5 0.5 2.8 - - 1.0 - 0.3 ++ ++ 1.0 1.0 0.3 - - 1.5 - less than 0.1 ++ + 1.5 1.5 less than 0.1 - - 2.0 - less than 0.1 +++ ++ 2.0 3.0 less than 0.1 + - In the production process according to the present invention, for example, Fe, Co, Ni, Mn, Cr, etc. or alloys of these metals are generally used as the solvent metal. As the carbon source in the present invention, there can be used synthetic or natural diamond powders and graphite powders or moldings thereof. As the seed crystal, small crystals of synthetic or natural diamond are used. In the present invention, furthermore, it is effective for the purpose of stabilizing the initial growth state of a crystal to arrange a buffer material consisting of at least one element selected from Al, Ni, Cu, Zn, Ga, Ag, Cd, In , Sn, Au, Tl and Pb, for example, Al plate or Cu plate between the surface of a seed and a solvent during synthesis of diamond. The plate thickness of the buffer material is generally in the range of about 0.01 to 0.5 mm. Thus, crystal defects or strains in the crystal due to unstable growth at the initial period of crystal growth can be reduced. The strains due to incorporation of the inclusions or the crystal defects or strains in the crystal due to unstable growth at the initial period of crystal growth can be reduced by employing these procedures. In the present invention, when diamond is synthesized at the commonly used ultra-high pressure and high temperature in the technical field of synthesizing diamond by the temperature gradient method and thereafter, the temperature and pressure at a sample section are lowered to normal temperature and pressure, the pressure is preferably released at an internal temperature of 300 to 1000 °C, preferably 400 to 800 °C, more preferably 500 to 600 °C, whereby to reduce remaining of strains due to stress in the crystal. In a preferred embodiment of the present invention for the process for the production of a strain-free synthetic diamond, the nitrogen getter is added to the solvent in a proportion of 0.1 to 5 weight %. Advantages of the Invention As illustrated above, the synthetic diamond of the present invention has less impurities, strains and crystal defects and reduced disadvantages such as cracking or breakage during compressing, and can be used as a diamond anvil for producing an ultra-high pressure or diamond anvil for FT-IR with a largely improved service life and stability. The synthetic diamond of the present invention can also be applied to uses needing high crystallinity of diamond, for example, monochromators, window materials for radiation light or radiation beam, semiconductor substrates, etc. In addition, the synthetic diamond of the present invention contains nitrogen and boron as impurities, but the contents of the both are substantially the same and compensated with each other, so the synthetic diamond of the present invention meets with no absorption of light due to nitrogen and boron and little strain in the crystal in an analogous manner to high purity diamond of IIa type hardly containing impurities. Thus, the synthetic diamond of the present invention having disadvantages such as cracking or breakage rduced during compressing can be used as a diamond anvil for producing an ultrahigh pressure or diamond anvil for FT-IR with a largely improved service life and stability. According to the present invention, diamond having very high crystallinity and light transmission property can be synthesized, which can be applied to uses such as monochromators, semiconductor substrates, jewels, etc. The above described high quality synthetic diamond can be realized by the production process of the present invention. Moreover, according to the method of measuring the strain of the synthetic diamond of the present invention, a more precise and quantitative measurement of strain than in the prior art is rendered possible. This is very advantageous. On the other hand, complete removal of nitrogen and boron has been considered very difficult in the prior art, but according to the production process of the present invention, a diamond crystal having substantially the same degree of light transmission property and crystallinity as high purity natural diamond of IIa type can readily be synthesized by the simple method comprising controlling the amounts of a nitrogen getter and boron to be added even under such a state that nitrogen and boron remain to some extent in the crystal. Therefore, the present invention is very advantageous in the progress of the industry. Examples The following examples are given in order to illustrate the present invention in greater detail without limiting the same. Example 1 In Fig. 1 is shown the structure of a sample chamber for the synthesis of diamond, used in Examples and Comparative Examples. As a carbon source 1, there was used synthetic diamond powder having a boron content of 3 ppm and as a solvent metal 2, there was used Fe and Co having a boron content of at most 1 ppm with a solvent metal composition of Fe/Co = 60/40 (weight ratio). To this solvent metal were added 1.5 weight % of Ti as a nitrogen getter (based on the weight of the solvent metal) and 1 weight % of Cu (based on the weight of the solvent metal). As a seed crystal 3, there was used a diamond crystal with a size of 500 µm. The carbon source 1 and seed crystal 3 were set in a graphite heater 5 to give a temperature gradient of about 30 °C and maintained at a pressure of 5.5 GPa and a temperature of 1300 °C for 70 hours in an ultra-high pressure producing apparatus to grow diamond on the seed crystal. Then, the temperature was lowered to room temperature and the pressure was reduced to take out a synthetic diamond. Consequently, there was obtained a colorless, transparent and good quality diamond crystal of IIa type, substantially free from inclusions, with 0.7 to 0.9 carat, which had a nitrogen content in the crystal of at most 0.1 ppm, measured by ESR. When the spectra of ultraviolet, visible and infrared were measured, other absorptions by nitrogen and boron were not found than that of the diamond itself. Observation of the resulting diamond by a polarizing microscope to observe a polarized light image and to estimate the strains tell that there was hardly found strain. The FWHM of the X-ray diffraction rocking curve was 5.8 arcseconds in the case of measuring using CuKα ray by the double crystal method with an arrangement in parallel to (004) plane of a synthetic diamond crystal as a first crystal. The FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum of diamond, masured by means of an apparatus for double monochromator Raman spectroscopic analysis with a resolving power of 0.5 cm -1 , was 1.8 cm -1 . Examples 2 to 4 and Comparative Example 1 As the carbon source 1, a synthetic diamond powder containing 23 ppm of boron was used and the amounts of Ti and Cu added to the solvent metal (based on the weight of the solvent metal) were changed as described below: Example 2: Ti 1.5 weight %, Cu 2 weight % Example 3: Ti 1.0 weight %, Cu 1 weight % Example 4: Ti 0.5 weight %, Cu 1 weight % Comparative Example 1: no addition of Ti and Cu Diamond crystals were synthesized in the similar manner to Example 1 but changing the amounts of Ti and Cu as described above. Four kinds of the thus resulting crystals, six natural diamond crystals of IIa type and six natural diamond crystals of Ia type were respectively subjected to measurement and estimation of nitrogen impurity, boron impurity, the FWHM of rocking curve, the FWHM of Raman peak and the strains by observation of polarized light images in an analogous manner to Example 1. The results are shown in Table 1 with the results of Example 1. Sample No. FWHM of X-ray Diffraction Rocking Curve (arcsec) FWHM of Raman Spectrum of 1332 to 1333 cmn -1 Peak (cmc -1 ) Amount of Strains by Polarized Image Amounts of N and B Impurities N B Example 1 5.8 1.8 no < 0.1 ppm < 0.1 ppm Example 2 6.1 2.1 little < 0.1 ppm ca. 1 ppm Example 3 7.2 2.3 little ca. 1 ppm ca. 1 ppm Example 4 8.3 2.3 little ca. 10 ppm ca. 1 ppm Comparison Example 1 20 2.5 somewhat much ca. 80 ppm ca. 1 ppm Natural Ia 200 ∼ 600 2.8 ∼ 8.3 much ca. 1000 ppm Natural IIa 300 ∼ 3000 2.2 ∼ 3.5 much < 1 ppm < 1 ppm Example 5 Diamond of IIa type was synthesized in an analogous manner to Example 1 except that an Al plate with a thickness of 0.05 mm, as a buffer material for stabilizing the initial crystal growth, was arranged between the solvent metal and seed crystal. Consequently, the crystallinity of the resulting diamond was further improved as represented by the fact that the FWHM of rocking curve was 5.6 arcsec. and the FWHM of Raman spectrum peak was 1.6 cm -1 . Example 6 Diamond of IIa type was synthesized in an analogous manner to Example 1 except that after the synthesis of diamond, lowering of the temperature and reduction of the pressure were simultaneously carried out and the lowering of the pressure was completed when the internal temperature was 500 °C. Consequently, the crystallinity of the resulting diamond was further improved as represented by the fact that the FWHM of rocking curve was 5.7 arcsec. and the FWHM of Raman spectrum peak was 1.6 cm -1 . In the above described Examples and Comparative Examples, (004) plane of diamond was used as a first crystal in X-ray diffraction and arranged in parallel, but in the method of measuring the strains by X-ray diffraction, another plane, for example, (111) plane of diamond can be used and it is also effective to arrange it otherwise, for example, in asymmetric manner. Example 7 In Fig. 1 is shown the structure of a sample chamber for the synthesis of diamond, used in Examples. As a carbon source 1, there was used synthetic diamond powder having a B (boron) content of 11 ppm and as a solvent metal 2, there was used Fe and Co having a B content of about 2 ppm with a solvent metal composition of Fe/Co = 60/40 (weight ratio). To this solvent metal were added 0.8 weight % of Ti as a nitrogen getter and simultaneously, 1 weight % of Cu. As a seed crystal 3, there was used a diamond crystal with a size of 500 µ m. The carbon source 1 and seed crystal 3 were set in a graphite heater 5 to give a temperature gradient of about 30°C between them and maintained at a pressure of 5.5 GPa and a temperature of 1300 °C for 70 hours in an ultra-high pressure producing apparatus to grow diamond on the seed crystal. Then, the temperature was lowered to room tempeature and the pressure was reduced to take out a synthetic diamond. Consequently, there was obtained a diamond crystal of IIa type, with 0.7 to 0.9 carat. The nitrogen and boron in the crystal were subjected to quantitative analysis to obtain 1.5 x 10 17 atoms/cm 3 and 1.5 x 10 17 atoms/cm 3 by SIMS. Observation of the resulting diamond by a polarizing microscope to observe a polarized light image and to estimate the strains taught that there was found a considerably small amount of strains. The FWHM of the X-ray diffraction rocking curve was 7.0 arcseconds in the case of measuring by the double crystal method with an arrangement in parallel to (004) plane of a synthetic diamond crystal as a first crystal. The FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum, masured by means of an apparatus for double monochromator Raman spectroscopic analysis was 2.2 cm -1 . Example 8 Diamond was synthesized in an analogous manner to Example 7 except changing the amounts of Ti and Cu added respectively to 1.5 weight % and 1.5 weight %. The resulting diamond was a blueish crystal which contained nitrogen and boron respectively in amounts of 1.0 x 10 16 atoms/cm 3 and 1.2 x 10 17 atoms/cm 3 by SIMS analysis. Measurement of the infrared absorption spectrum told that absorption by boron was found in the vicinity of 2800 cm -1 . Observation of the resulting diamond by a polarizing microscope to observe a polarized light image and to estimate the strains taught that there was found a small amount of strains. The FWHM of the X-ray diffraction rocking curve was 8.3 arcseconds in the case of measuring by the double crystal method with an arrangement in parallel to (004) plane of a synthetic diamond crystal as a first crystal. The FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum, masured by means of anapparatus for double monochromator Raman spectroscopic analysis was 2.5 cm -1 . Example 9 Diamond was synthesized in an analogous manner to Example 7 except adding 0.04 weight % (based on carbon source) of boron to the carbon source, 1.5 weight % of Al as a nitrogen getter and no copper. Consequently, there was obtained a diamond crystal of IIa type, with 0.7 to 0.9 carat. The nitrogen and boron in the crystal were subjected to quantitative analysis by SIMS to obtain 1.8 x 10 17 atoms/cm 3 and 1.5 x 10 17 atoms/cm 3 by SIMS. Observation of the resulting diamond by a polarizing microscope to observe a polarized light image and to estimate the strains taught that there was found a considerably small amount of strains. The FWHM of the X-ray diffraction rocking curve was 7.2 arcseconds in the case of measuring by the double crystal method with an arrangement in parallel to (004) plane of a synthetic diamond crystal as a first crystal. The FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum, masured by means of an apparatus for double monochromator Raman spectroscopic analysis, was 2.2 cm -1 . Example 10 Diamond was synthesized in an analogous manner to Example 9 except changing the amount of Al to 0.5 weight % . The resulting diamond was a somewhat yellowish crystal which contained nitrogen and boron respectively in amounts of 1.6 x 10 16 atoms/cm 3 and 1.5 x 10 17 atoms/cm 3 by SIMS analysis. Measurement of the infrared absorption spectrum and ultraviolet-visible spectrum as to the resulting diamond told that in any case, absorption of nitrogen was found. Observation of the resulting diamond by a polarizing microscope to observe a polarized light image and to estimate the strains taught that there was found a relatively small amount of strains. The FWHM of the X-ray diffraction rocking curve was 8.5 arcseconds in the case of measuring by the double crystal method with an arrangement in parallel to (004) plane of a synthetic diamond crystal as a first crystal. The FWHM of a peak at 1332 cm -1 to 1333 cm -1 in the Raman spectrum, measured by means of an apparatus for double monochromator Raman spectroscopic analysis was 2.5 cm -1 . Example 11 As a carbon source 1, there was used synthetic diamond powder having a B (boron) content of 7 ppm and as a solvent metal 2, there was used Fe and Co having a B content of about 1 ppm with a solvent metal composition of Fe/Co = 60/40 (weight ratio). To this solvent metal were added 1.5 weight % of Ti as a nitrogen getter and simultaneously, 1.5 weight % of Cu. As a seed crystal 3, there was used a diamond crystal with a size of 500 µ m. The other procedures were rendered similar to those of Example 7 to synthesize diamond of IIa type. Consequently, there was obtained a colorless, transparent and good quality diamond crystal of IIa type, with 0.7 to 0.9 carat. The nitrogen and boron in the crystal were subjected to quantitative analysis by SIMS to obtain 1.1 x 10 16 atoms/ cm 3 and 1.8 x 10 16 atoms/cm 3 . Observation of the resulting diamond by a polarizing microscope to observe a polarized light image and to estimate the strains taught that there was hardly found strains in the crystal. The FWHM of the X-ray diffraction rocking curve was 5.8 arcseconds in the case of measuring by the double crystal method with an arrangement in parallel to (004) plane of a synthetic diamond crystal as a first crystal. The FWHM of a peak at 1332 to 1333 cm -1 in the Ramanspectrum, measured by means of an apparatus for double monochromator Raman spectroscopic analysis was 1.8 cm -1 . Thus, this was found to be a diamond crystal very excellent in crystallinity. Measurement of the ultraviolet-visible spectrum and infrared spectrum thereof gave the result that no absorption of light was found due to the impurities such as nitrogen and boron. According to the expert's evaluation of the color grade as a gem diamond, it was F color based on GIA scale, which is a grade corresponding to the highest grade of natural high purity diamond. Example 12 Diamond of IIa type was synthesized in an analogous manner to Example 11 except that an Al plate with a thickness of 0.05 mm, as a buffer material for stabilizing the initial crystal growth, was arranged between the solvent metal and seed crystal. Consequently, the crystallinity of the resulting diamond was further improved as represented by the fact that the FWHM of rocking curve was 5.7 arcsec. and the FWHM of Raman spectrum peak was 1.6 cm -1 The other properties were same as those of Example 11. Example 13 Diamond of IIa type was synthesized in an analogous manner to Example 11 except that after the synthesis of diamond, lowering of the temperature and reduction of the pressure were simultaneously carried out and the lowering of the pressure was completed when the internal temperature was 500 °C. Consequently, the crystallinity of the resulting diamond was further improved as represented by the fact that the FWHM of rocking curve was 5.7 arcsec. and the FWHM of Raman spectrum peak was 1.6 cm -1 . The other properties were same as those of Example 11.

Claims: A synthetic diamond, in which the nitrogen content is at the most 10 ppm and the boron content is at the most 1 ppm and in which an X-ray diffraction analysis by the double crystal method, in which a diamond crystal is used as a first crystal, the FWHM of the X-ray diffraction rocking curve is at the most 10 arcseconds for measurement using CuKα rays with an arrangement in parallel to the (004) plane. A synthetic diamond as claimed in claim 1, wherein the nitrogen content is at the most 0.1 ppm. A synthetic diamond as claimed in claim 1 or claim 2, wherein the boron content is at the most 0.1 ppm. A synthetic diamond as claimed in any one of the preceding claims, in which the FWHM of a peak at 1332 to 1333 cm -1 in the Raman spectrum of diamond, measured by means of an apparatus for Raman spectroscopic analysis with a resolving power of at the most 1 cm -1 , is at the most 2.3 cm -1 , preferably at the most 2 cm -1 . A synthetic diamond as claimed in claim 1, in which the nitrogen content is at the most 0.1 ppm and the boron content is at the most 0.1 ppm and in an X-ray diffraction analysis by the double crystal method, in which a diamond crystal is used as the first crystal, the FWHM of the X-ray diffraction rocking curve is at the most 6 arcseconds for measurement by CuKα rays with an arrangement in parallel to the (004) plane. A synthetic diamond as claimed in claim 5, in which the FWHM of a peak at 1332 to 1333 cm -1 in the Raman spectrum of diamond, measured by means of an apparatus for Raman spectroscopic analysis with a resolving power of at the most 1 cm -1 , is at the most 2 cm -1 . A synthetic diamond as claimed in any one of the preceding claims, in which nitrogen atoms and boron atoms are contained in the crystal and the difference between the number of the nitrogen atoms and that of the boron atoms is at the most 1 x 10 17 atoms/cm 3 , preferably at the most 1 x 10 16 atoms/cm 3 . A process for the production of a synthetic diamond as claimed in any one of claims 1 to 6 by the temperature gradient method employing a carbon source, a solvent metal and a seed crystal, wherein the carbon source is separated from the seed crystal by the solvent metal and a temperature gradient exists between the carbon source and seed crystal at an ultra-high pressure and a high temperature, which comprises using a carbon source having a boron content of at the most 10 ppm and a solvent metal having a boron content of at the most 1 ppm and adding a nitrogen getter to the solvent metal. A process for the production of a synthetic diamond as claimed in claim 8, wherein graphite, which has been subjected to a halogenising treatment in order to reduce the boron of the graphite to at the most 1 ppm, is used as the carbon source. A process for the production of a synthetic diamond as claimed in claim 8 or claim 9, wherein the nitrogen getter is at least one element from Group IVa or Va of the Periodic Table. A process for the production of a synthetic diamond as claimed in any one of claims 8 to 10, wherein at least one material capable of preventing carbide formation of the nitrogen getter is added to the solvent metal. A process for the production of a synthetic diamond as claimed in any one of claims 8 to 11, wherein a buffer material capable of stabilizing the initial growth of the crystal is arranged between the surface of a seed crystal and the solvent metal. A process for the production of a synthetic diamond as claimed in any one of claims 8 to 12, wherein diamond is synthesized at an ultra-high pressure and high temperature, and wherein the temperature and pressure are reduced after synthesis to normal temperature and pressure, and the reduction in pressure is completed at a temperature in the range of from 300 to 1000°C. A process for the production of a synthetic diamond as claimed in claim 7 by the temperature gradient method employing a carbon source, a solvent metal and a seed crystal, wherein the carbon source is separated from the seed crystal by the solvent metal and a temperature gradient exists between the carbon source and seed-crystal at an ultra-high pressure and a high temperature, which comprises controlling the amount of a nitrogen getter added to a solvent metal and/or controlling the amount of boron added to a carbon source and/or a solvent metal so that the number of nitrogen atoms incorporated into the crystal is substantially the same as the number of boron atoms incorporated into the crystal during synthesis. A process for the production of a synthetic diamond as claimed in claim 14, wherein Al is used as the nitrogen getter. A process for the production of a synthetic diamond as claimed in claim 14 or 15, wherein the nitrogen getter if at least one element of Group IVa or Va of the Periodic Table. A process for the production of a synthetic diamond as claimed in any one of claims 14 to 16, wherein at least one material capable of preventing carbide formation of the nitrogen getter is present. A process for the production of a synthetic diamond as claimed in any one of claims 14 to 17, wherein a buffer material capable of stabilizing the initial growth of the crystal is arranged between the surface of a seed crystal and the solvent metal. A process for the production of a synthetic diamond as claimed in claim 18, wherein the buffer material is at least one of Al, Ni, Cu, Zn, Ga, Ag, Cd, In, Sn, Au, Tl or Pb. A process for the production of a synthetic diamond as claimed in any one of claims 14 to 19, wherein diamond is synthesized at an ultra-high pressure and high temperature, and wherein the temperature and pressure are reduced after synthesis to normal temperature and pressure, and the reduction in pressure is completed at a temperature in the range of from 300 to 1000°C.

Inventors: NISHIBAYASHI YOSHIKI, SATOH SHUICHI, SUMIYA HITOSHI, NISHIBAYASHI, YOSHIKI, SATOH, SHUICHI, SUMIYA, HITOSHI, NISHIBAYASHI, YOSHIKI, C/O ITAMI WORKS OF, SATOH, SHUICHI, C/O ITAMI WORKS OF, SUMIYA, HITOSHI, C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1994
Main classification: C01B31
IPC codes: C01B33, C01B31, B01J3